Add a minimal path manipulation C module.

This is just for the VM's own internal use, for resolving relative
imports.

Also added a tiny unit test framework for writing tests of low-level
C functionality that isn't exposed directly by the language or VM.

test/unit/test.h

@@ -0,0 +1,7 @@
+// Set this to 1 to output passing tests.
+#define SHOW_PASSES 0
+
+void pass();
+void fail();
+
+int showTestResults();